The present invention relates to improvements in high performance absorbance detectors and more particularly to such an improved detector including a flashlamp and a compact folded optics system.
U.S. Pat. Nos. 3,810,696 and 4,565,447 describe light absorbance detectors including flashlamps as a pulsed source of light. The Model 490 Multiwavelength HPLC Detector of the Waters Chromatography Division of the Millipore Corporation, Milford, Mass. is a commercial embodiment of the detector described in the '447 patent.
While there are well recognized advantages associated with the use of flashlamps in absorbance detectors relative to power requirements, lamp life, system warm up time and breath of light spectrum, there are also well recognized limitations particularly when it is desired to reduce the noise in such systems to the theoretical shot noise level. In particular, it is recognized that the noise generated in absorbance detectors including flashlamps such as the Waters Model 490 is significantly greater than similar detectors including deuterium lamps and several times greater than the theoretical shot noise level. The flashlamp is the principal source of such greater levels of noise.
In developing the soon to be released, improved performance absorbance detector, the DYNAMAX MODEL UV-1 of the Rainin Instrument Co., Inc., the inventors of the subject matter set forth in this application isolated five primary characteristics of flashlamps which contribute to absorbance noise in high sensitivity absorbance detectors. They are:
(1) light amplitude variations flash to flash,
(2) flashlamp arc position variations from flash to flash,
(3) spatial intensity variations across the surface of the flashlamp arc from flash to flash,
(4) angular light pattern variations from flash to flash, and
(5) light spectrum variations from flash to flash.
In the DYNAMAX MODEL UV-1, each of the foregoing noise characteristics is minimized to produce an absorbance detector having a noise specification of less than 2.times.10.sup.-5 AU. The first of the above-listed noise characteristics may be compensated for by improved electronics while optics with low spatial sensitivity and improved flow cell design may minimize the effects of the second and third noise characteristics. In accordance with the present invention, noise associated with the third, fourth and fifth noise characteristics are substantially eliminated by the inclusion of a unique reflecting beam splitter with highly polished mirror surfaces and splitter and flow cell masks in the optics system of the DYNAMAX MODEL UV-1.
As to angular pattern variations in consecutive flashes from a flashlamp, since the flashlamp arc is not totally transparent and has spatial variations in its brightness, the normalized radiation intensity in different directions will vary from flash to flash. The relative intensity variations in two different directions is approximately proportional to the angular separation between the two directions for small angles. The typical amplitude of the angular noise is such that two beams separated by an angle of 0.3 degrees results in a filtered absorbance noise of approximately 1.0.times.10.sup.-5 AU peak-to-peak for a one second rise time and a 3 degree separation results in a filtered absorbance noise of approximately 10.times.10.sup.-5 AU peak-to-peak.
As to flash to flash spectral variations, the relative intensity of the light as a function of wavelength varies slightly from flash to flash. This has the effect of varying the center wavelength and the spectral shape of the bandpass of the monochromator output of the absorbance detector especially at monitored wavelengths where the light intensity changes rapidly as a function of wavelength. That is, on the sides or shoulders of a spectral peak.
The Waters Model 490 Absorbance Detector virtually eliminates noise due to angular flash pattern variations from the flashlamp source but is severely affected by spectral variations flash to flash. In these regards, the Waters Model 490 and the absorbance detector described in the '447 patent include a beam splitter which intercepts a single beam of light with a semitransparent material that reflects a portion of the beam in one direction and transmits another portion in a different direction. Such beam splitters are commonly made of quartz which reflects on the order of 10% of the incident light and transmits the rest. For quartz coated with a material such as inconel which reflects a greater percentage of light the result may be two beams of similar intensities. Such beam splitters are available from many sources such as the Oriel Corporation of Stratford, Conn.and are described in its catalog entitled "Optics & Filters". In the Waters Model 490, the two resulting beams from such a splitter view the light source essentially in the same direction. Viewing the source from essentially the same direction virtually eliminates noise due to angular light pattern variations from the flashlamp source. Unfortunately however, this type of beam splitter is wavelength sensitive. In particular, the ratio of the intensities of the two resulting beams, the "split ratio", varies as a function of wavelength. The wavelength sensitivity of the split ratio when combined with the flash to flash spectrum variations from the flashlamp causes noise which at many wavelengths over the detected spectrum of wavelengths is considerably larger than the shot noise level.
Accordingly, there are continuing needs for an absorbance detector with flashlamp source and high light throughput which is characterized by a low noise level approaching that of shot noise and which is unaffected by angular pattern and spectral variations flash to flash. The present invention satisfies such needs.